The present invention relates to a glass substrate for a thin film magnetic data storage disc and to a process for producing such a glass substrate.
Thin film magnetic discs for storing data are known in which a magnetic recording medium has been deposited onto a glass substrate. In use, the magnetic disc is disposed in a magnetic disc drive unit which utilises a magnetic head for reading and writing information from and onto the disc. Prior to the reading and writing operation, the disc is stationary and the head is in contact with the magnetic recording medium on the disc. In the reading and writing operation, the disc is driven to rotate at a predetermined speed relative to the head which can be moved radially relative to the disc. The head is spaced a very small distance from the magnetic recording medium and is adapted to read information from and write information onto the magnetic recording medium at the desired place on the magnetic recording medium. After the reading and writing operation, the rotation of the disc is terminated and the head is permitted to return to the initial configuration wherein the head contacts the disc. This method of operating a disc is known in the art as a contact-start-stop (CSS) method. It is known that such magnetic disc drive units suffer from the problem of "stiction" or static friction which exists between the head and the magnetic recording medium on initiation or termination of the reading or writing operation. This stiction can cause wear of the head and of the medium and thus can reduce the life time of the magnetic disc drive unit. The tendency for stiction to occur increases with increasing smoothness of the magnetic recording medium. Glass is known for use as a substrate for such magnetic discs, although it is also well known to use aluminium discs as the substrate, because glass is hard and has high surface smoothness. However, glass can suffer from the disadvantage that it has such a smooth surface that the overlying magnetic recording medium which has a correspondingly smooth surface is so smooth that the surface area of the medium actually contacting the static recording head is sufficiently large whereby the stiction problem can be encountered during use of the drive unit. This can cause premature failure of the drive unit and/or the media as described above.
It is known from U.S. Pat. No. 4,833,001 to provide a glass substrate for a magnetic disc with an isotropically roughened surface. The surface roughening is achieved by chemical etching of the glass surface either by a gas containing hydrogen fluoride or by an aqueous solution containing a fluorine compound. U.S. Pat. No. 5,087,481 discloses a method for texturing a magnetic disc silicate glass substrate in which prior to surface roughening or "texturing" of the disc, this being achieved by chemical etching, the glass surface is subjected to a chemical or gentle mechanical polishing step to remove microcracks or scratches in the disc surface. However, we have found that the use of chemical etching of the disc surface has the disadvantage that a fully isotropic micro-textured surface can be difficult to achieve.